how to add texture to an object import in .stl format
An image or procedural texture is used by a material to control the color over the surface of the object.

Using blender render, a texture can only be added if a material has been assigned to the object. Also note that the texture panel has sub-sections, you will want the material textures option to be selected. After importing an .stl file, there may be no material, so first create the material and then add a texture.



Also there are differences based on the render engine chosen, cycles uses a nodes based system that allows textures to be incorporated at different stages of the material.